The present invention relates to a cutting insert with an entirely or partly threaded central hole for receiving a locking screw.
The vast majority of the plurality of different cutting inserts existing today on the market have a central through hole. This hole is intended to accommodate a suitable locking screw for fastening the insert in a holder. If the cutting insert is intended for turning, the holder usually consists of a turning bar; if the cutting insert is intended for milling, the holder is, e.g., a milling cutter body, a long edge milling cutter or an end mill. The cutting insert may also be intended for boring and then it may be fastened by a screw in a boring shaft. Most commonly the cutting insert is an indexable cutting insert, which means that when a cutting edge has been worn out, the locking screw may be loosened and the insert turned by a half or quarter of a revolution, whereby a new cutting edge is placed or indexed into an operative position. Cutting inserts including indexable, cutting inserts exist with innumerous different geometries, in order to give a good chip breaking and chip control and a low energy consumption. The inserts are mostly made of coated or uncoated cemented carbide, but cutting inserts of different ceramic materials also exist. They usually have a square, triangular, rhombic, rectangular or hexagonal basic shape, but other basic shapes also occur. However, they usually have one thing in common, namely that they are all provided with a central through hole in the geometrical central point. These holes can have different hole designs, but they all have in common that the hole wall is substantially smooth. As mentioned, a threaded screw is inserted into this smooth hole and is threaded into a threaded hole in the holder per se, upon which the insert rests, and is fastened onto this holder. This basic configuration usually functions satisfactorily, but is nevertheless marred by some drawbacks, which it would be desirable to eliminate. For instance, the screw head must be countersunk into the insert's hole in order not to obstruct the chip flow, which brings about a further weakening of the cutting insert, which is already relatively fragile. Further, specifically because of this weakening of the insert's strength, the central hole cannot be made with too a large diameter, which means that the screw's diameter is relatively weak. Therefore, it sometimes occurs that the screw is broken when tightened with out care and/or too strongly. Seen from another aspect, the small maximum allowed screw diameter restricts the possible magnitude of the tightening strength.
Another disadvantage is that the requirement of accessability to the screw head sometimes constitutes a complicating and restricting factor for certain application e.g., for extremely close pitch cutters. Another case when good accessability is desirable is when a very large number of cutting inserts are to be screw-fastened by a power-driven screw tightener with a preset maximal torque. For instance, in some bar peeling operations the number of cutting inserts is very high and the accessability for such a screw tightener is poor.
Thus, one object of the present invention is to provide a cutting insert with a larger hole diameter in order to make possible larger tightening forces.
Another object of the present invention is to improve the accessability when tightening the locking screw.